Food preparation apparatus wherein heat is transferred from a heat source to a food preparation surface by a vaporized working fluid is disclosed and claimed in an earlier application for Letters Patent Ser. No. 165,569 of Lazaridis, Searight and Shefsiek, filed July 23, 1971, now abandoned, which is assigned to the same assignee as the present application. As is explained in that earlier application, a heat transfer liquid is confined in a sealed enclosure which extends from the heat source to the food preparation surface. The heat transfer liquid is vaporized at a surface adjacent the heat source and fills the sealed enclosure. It then condenses on a portion of the sealed enclosure adjacent the food preparation surface and releases heat energy. The apparatus used in this form of heat transfer is sometimes called a "heat pipe". Advantages of this mode of food preparation over prior modes involving use of heat sources applied directly to cooking surfaces or to foodstuffs are stated in the earlier application.
Deep fat frying of foodstuffs is done essentially by immersing the foodstuffs in a vat of heated oil. Deep fat frying equipment that is presently available involves the application of a heat source directly to one side of a partition the opposite side of which is in direct contact with the frying oil. For example, many gas-fired fryers have burner tubes running through the oil fat. These tubes ordinarily have a burner at one end and combustion products travel from the burner through the tube so that heat is transferred from the tube to the oil in the vat. The tubes are understandably hotter at the burner end than at the exhaust end. Although this may result in an average temperature which satisfies the requirements for heat transfer from the tube to the oil, it unavoidably creates, at the hot end, a temperature sufficiently high to produce a substantially larger amount of oil breakdown than that which would be associated with a temperature equivalent to the average temperature of the burner tubes. Further, a vat fitted with electric heating coils or elements will heat oil contained in it only in the vicinity close to the heating elements. Such electric elements tend to overheat the cooking oil in this close vicinity. Most efficient use of electric elements occurs when they are immersed in the cooking oil vat where they require significant space and create objectionable cleaning problems. In the absence of either immersed gas tubes or immersion electric heaters, the usual procedure is just to expose a pot or vat to direct flame.
There must be balance, or "trade-off" of two conflicting parameters, one against the other. On the one hand, it is desirable to cook the food at as high a temperature as possible in order to avoid excessive absorption of the oil by the food being cooked; e.g.: in order to have a relatively dry and crisp donut, instead of a soggy, oily or greasy donut. On the other hand, it is advantageous to maintain the cooking oil at as low a temperature as possible in order to minimize the formation of undesirable reaction products in the oil.
Deep fat frying equipment that is currently available does not achieve desired temperature uniformity and control. The formation of undesirable reaction products of frying oil is a problem which exists with currently available apparatus. This problem is exacerbated by the occurence of hot spots, or relatively hot zones, which may be from any one of several sources, not limited to the inherent properties of existing heating methods. One theory concerning the deleterious effects of hot spots in deep fat fryers suggests that particles of food materials and the like may adhere to the heat transfer surface inside the vat, and act there as a thermal insulator so that heat is not transferred from the wall to the oil. In addition, oil held by these particles cannot move away from the "insulated" spots becoming hotter and causing this oil and oil in contact with it to decompose. In the best of prior situations, the mechanical process of such heating is based on localized over-heating of cooking oil which then heats the contents by convection.
Deterioration of cooking oil is temperature dependent. It occurs continuously as a function of temperature, usually an exponential function, the rate at which undesirable reaction products are produced increasing exponentially in relation to temperature rise.
Generally, chemical breakdown of thermally unstable liquid oils is exponential with increasing temperature, and it is predictable that oxidation or other decomposition at localized hot spots will far exceed the decomposition at normal cooking temperatures. If, for example, localized hot spots exceed the average surface temperature by 25.degree. F or more, they will contribute a disproportionally high amount of thermal decomposition.
Replacement of oil that has formed an excess of undesirable reaction products is both time consuming and costly. It necessitates, among other things, that the vat be cleaned, and this task is not easy when the vat includes burner tubes or immersion heaters. It is apparent that these tubes make it difficult to clean the vat around and under them.